Vicudu tells you the difference between a shielded room and a dark room

The shielding room is just a large iron box that blocks the radio signals inside and outside. Simply put, the inside cannot go out and the outside cannot come in, but the electromagnetic waves inside will reflect and stack on the inner wal

An anechoic chamber is based on a shielding chamber, with absorbing materials laid on the inner wall to simulate the effect of an open field. The anechoic chamber is much more expensive than a shielding chamber, but it is more expensive to stick these materials inside the anechoic chamber. The electromagnetic waves inside will be absorbed when emitted to the inner wall, and there will be basically no mixed wave effect of reflection and superposition. Suitable for testing the radiation emission interference of samples. Anechoic chambers are generally divided into full anechoic chambers and semi anechoic chambers.

Tests that do not have high requirements for the experimental environment, such as conducted disturbances, static electricity tests, surge tests, lightning tests, etc., are all conducted through power lines, so they only need to be conducted in a shielded room; For spatial radiation, spatial disturbance transmitted through space, or anti-interference, there are special requirements for space, so it needs to be carried out in a dark room to simulate the space of an open field.

Radiation tests conducted in full anechoic chamber, semi anechoic chamber, and open field can generally be considered to conform to the propagation laws of electromagnetic waves in free space.

The full anechoic chamber reduces the interference of external electromagnetic wave signals on the test signal, and electromagnetic wave absorbing materials can reduce the multipath effect caused by the reflection of walls and ceilings on the test results. It is suitable for emission, sensitivity, and immunity experiments. In practical use, if the shielding effectiveness of the shielding body can reach 80dB~140dB, then the interference to the external environment can be ignored. In a fully anechoic chamber, the situation in free space can be simulated. Compared with the other two testing sites, the floor, ceiling, and wall of the full anechoic chamber have the smallest reflection, are least affected by external environmental interference, and are not affected by external weather. Its disadvantage lies in cost constraints and limited testing space.

Semi anechoic chamber

A semi anechoic chamber, similar to a full anechoic chamber, is also a shielded six sided box body covered with electromagnetic wave absorbing material inside. The difference is that the semi anechoic chamber uses a conductive floor and does not cover the absorbing material. A semi anechoic chamber simulates an ideal open field situation, where the field has an infinitely large and well conductive ground plane. In a semi anechoic chamber, as the ground is not covered with absorbing materials, a reflection path will be generated, so the signal received by the receiving antenna will be the sum of the direct path and reflection path signals.

The shielding room is a metal enclosure with a large number of resonant frequencies. Once the radiation frequency and excitation mode of the tested equipment cause the shielding room to resonate, the measurement error can reach 20~30dB, so it is necessary to install wave absorbing materials on the surrounding walls and top of the shielding room to greatly weaken the reflection, that is, only direct waves and ground reflected waves are available for radio propagation, and its structural dimensions are also based on the requirements of the open test field, Thus, it can simulate outdoor open field testing, which is an electromagnetic shielding and absorbing anechoic chamber, also known as an EMC anechoic chamber.

The EMC darkroom structure is usually composed of several parts such as an RF shielding chamber, absorbing materials, power supply, antenna, turntable, etc.: the RF shielding chamber ensures that the test is not affected by external interference; Ensure the absorption characteristics of the darkroom with absorbent materials; Antennas and turntables ensure that the tested object is tested according to the state and conditions required by the standard; The power supply system ensures the use of electricity for the test. Auxiliary equipment such as RF shielded doors, ventilation waveguide windows, cameras, lighting fixtures, power boxes, etc. should be designed to be placed outside the main reflection area as much as possible to avoid any metal parts being exposed in the main reflection area.

The floor of the darkroom is the only reflecting surface of electromagnetic waves. The requirement for the floor is: continuous, flat, and without bumps. There should be no gaps exceeding 1/10 of the minimum working wavelength to maintain the conductivity continuity of the floor. The grounding wire and power cord in the darkroom should be laid at the foot of the wall, and should not cross the room. The wires should also be threaded through metal pipes, and the metal pipes should be well overlapped with the floor. To avoid the impact of radio wave reflection on measurement errors, personnel and testing control equipment should not be located in the testing site. So generally, EMC darkrooms are composed of a testing darkroom and a control room, where testing antennas and tested equipment are placed, and operators and testing control instruments are located in the control room. If there are high-power amplifier devices, an amplifier room should also be established to place these devices to avoid interference with the surrounding environment. The darkroom and control room should each adopt independent power supply systems, using different phase power supplies and passing through their respective filters to avoid interference from the control room entering the darkroom through power lines.